This application focuses on peroxisome proliferators, a chemically diverse group of compounds encountered as nutrients, clinical drugs and industrial chemical toxins. Effects of dermal exposure to peroxisome proliferators may range from epidermal hypertrophy to defects in differentiation and keratinization; the effects of these compounds on keratinocytes have not been rigorously investigated. Inquiry as to the mechanisms of effects in keratinocytes is aided by the recent characterization of a transcription factor, the peroxisome proliferator activated receptor (PPAR) which is a member of the steroid/thyroid hormone nuclear receptor superfamily. Its stimulation by long chain fatty acids is particularly relevant to keratinocytes as these compounds are abundant in epidermis. The long-term goals of the proposal are to understand how peroxisome proliferators modulate the balance of cellular replication and terminal differentiation in epidermal keratinocytes and if their activity is directly mediated by the nuclear receptor. Initial studies of keratinocyte expression of PPAR and differentiation response to peroxisome proliferators has recently been completed. This has been compared to the effects of retinoic acid (RA). RA suppresses keratinocyte differentiation and is mediated by retinoic acid receptors (RAR). Initial investigations show PPAR stimulation in keratinocytes coincides with changes in RAR activity suggesting gene regulation by PPAR. Peroxisome proliferators of increasing potency will be tested in keratinocytes to see if the differentiation effect parallels receptor activity. Keratinocytes in standard and organotypic cultures will be examined for changes in growth versus expression of differentiation by analysis of mitotic activity versus involucrin, a marker of maturing, postmitotic keratinocytes. The role of PPAR in differentiation will be examined at the mRNA level and by growing organotypic cultures of keratinocytes made deficient for PPAR mRNA via introduction of antisense constructs or oligonucleotides. Possible reasons for different RAR activity, e.g., retinoic acid degradation or levels of RAR gene expression will be pursued. The apparent cross regulation between peroxisome proliferators and retinoic acid may be exploited for future clinical development in certain disease states (psoriasis, ichthyoses) or injuries (cutaneous burns, ulcers).